The invention relates to a charge storage device.
The invention has been developed primarily for super capacitors and will be described hereinafter with reference to that application. It will be appreciated, however, that the invention is not limited to that particular field of use and is also applicable to other charge storage devices.
Electric double layer capacitors store charge in the large surface area of the electric double layer which is formed between activated carbon and an electrolyte. Typically, activated carbon powder is combined with a fluoride resin and methanol to make a paste. The paste is spread on aluminium foil and allowed to dry, to form electrodes. The electrodes are wound with a separator, impregnated with electrolyte and sealed into a container. Terminals connected to respective, electrodes and extending outside the container complete the capacitor.
According to a first aspect of the invention there is provided a charge storage device including:
a first longitudinally extending sheet electrode;
a second longitudinally extending sheet electrode disposed adjacent to the first electrode and being folded together with the first electrode along a longitudinal fold line and a transverse fold line;
a porous separator disposed between adjacent electrodes: and
a sealed package for containing the electrodes, the separator and an electrolyte, whereby the first electrode is electrically connected to a first terminal and the second electrode is electrically connected to a second terminal, both the first and second terminals extending from the package to allow electrical connection to the respective electrodes.
According to a second aspect of the invention there is provided a charge storage device including:
a first longitudinally extending sheet electrode;
a second longitudinally extending sheet electrode disposed adjacent to the first electrode and being folded together with the first electrode along two spaced apart transverse fold lines;
a porous separator disposed between adjacent electrodes: and
a sealed package for containing the electrodes, the separator and an electrolyte, whereby the first electrode is electrically connected to a first terminal and the second electrode is electrically connected to a second terminal, both the first and second terminals extending from the package to allow electrical connection to the respective electrodes.
Preferably, in the second aspect, the first and second electrodes are folded together along a longitudinal fold line.
Preferably, the first and second electrodes are folded together along a plurality of spaced apart longitudinal fold lines and a plurality of spaced apart transverse fold lines. Even more preferably, the first and second electrodes are folded together along two spaced apart longitudinal fold lines and two spaced apart transverse fold lines in a Z configuration.
Preferably also, the first and the second sheet electrodes include respective outwardly extending tabs which are electrically connected to respective first and second terminals. More preferably, the tabs extend transversely outwardly from the respective electrodes. Even more preferably, the respective tabs extend outwardly away from each other.
In a preferred form each electrode includes two longitudinal edges and two transverse edges extending between the longitudinal edges wherein the tabs extend centrally outwardly from one of the respective longitudinal edges.
Preferably, a plurality of like pairs of first and second sheet electrodes, together with the intermediate separators, are disposed within the package and connected in parallel to the first and second terminals.
According to a third aspect of the invention there is provided a method of manufacturing a charge storage device including the steps of:
providing a first longitudinally extending sheet electrode;
disposing a second longitudinally extending sheet electrode adjacent to the first electrode;
folding the second electrode together with the first electrode along a longitudinal fold line and a transverse fold line;
disposing a porous separator between adjacent electrodes: and
sealing the electrodes, the separator and an electrolyte in a package such that the first electrode is electrically connected to a first terminal and the second electrode is electrically connected to a second terminal, whereby both the first and second terminals extend from the package to allow electrical connection to the respective electrodes.
According to a fourth aspect of the invention there is provided a method of manufacturing a charge storage device including the steps of:
providing a first longitudinally extending sheet electrode;
disposing a second longitudinally extending sheet electrode adjacent to the first electrode;
folding the second electrode together with the first electrode along two spaced apart transverse fold lines;
disposing a porous separator between adjacent electrodes: and
sealing the electrodes, the separator and an electrolyte in a package such that the first electrode is electrically connected to a first terminal and the second electrode is electrically connected to a second terminal, whereby both the first and second terminals extend from the package to allow electrical connection to the respective electrodes.
According to another aspect of the invention there is provided a method of manufacturing a charge storage device including the steps of:
(a) applying a coating containing a predetermined proportion of activated carbon, in a predetermined thickness, over an area which extends lengthways along a strip of conductive foil to form a strip electrode. The application may take place by printing, coating dry-applying and fusing. The coating may be a paste, ink or other compound. The coating will typically comprise a paste having a composition of about 80% activated carbon, about 20% conductive carbon and between 3 and 15% binder, but these proportions may vary widely. The coating will usually be between 1 and 500 microns thick, or more particularly between 10 and 300 microns thick, and it may be applied to one or both sides of the foil. The foil will likely be aluminium but could be any other suitable conductive material such as copper or a conductive plastics material. The foil or other conductive material may be pre-treated to enhance its conducting, adhesive or other properties.
(b) sandwiching an insulating separator between the coated regions on two strip electrodes to form a layered strip. The sandwich may be formed with any suitable insulating separator, depending of the type of electrolyte to be used. A porous membrane of plastics material will often be used. The two strips of foil may be shaped or placed such that part of each extends out of the dimensions of the sandwich to be conveniently accessible for connection to an electrical terminal;
(c) folding the layered strip lengthways to form a folded layered strip. The strip may be folded once to make a double thickness, or it may be folded several times. A double fold in the form of a xe2x80x9cZxe2x80x9d is currently considered to be a practical option. An alternative has coating spread over both sides along one edge of the strips of foil, and over one side of the other edge. The edges with coating over both sides are then overlapped with the insulating separator between them. The other edge of each strip has the coating lying against the separator and this edge is then folded over the other sheet in the region of the overlap;
(d) the layered strips may be produced in a batch process to predetermined lengths. Alternatively the layered strip may be produced continuously and then cut to the required lengths. In this case it is necessary to take precautions to ensure there are no short circuits. The process produces a long length of the layered strip in which the area of overlap of the pasted areas on the electrodes is related to the length, and to the capacitance. The strip is cut, either before or after it is folded lengthways, to give the required capacitance;
(e) folding the cut length crossways to form a twice folded structure. The second fold will usually be made after the strip to cut from the long length since this makes handling easier. The second fold will also usually be made after the first fold, but it is conceivable for the layered strip to be folded crossways and then folded lengthways. The double fold gives the structure a generally thin rectangular form;
(f) impregnating the structure with electrolyte. The structure will generally be placed in a container which will become the final package before impregnating it with electrolyte. The impregnation step in itself may follow the conventional procedure used with double layer capacitors, and any suitable electrolyte may be used;
(g) in one form electrical connection terminals may be attached to the electrodes. If the two strip electrodes are longitudinally offset the terminals may be attached to respective non-overlapping ends. In an alternative, the electrodes may have lateral extensions to which the terminals are attached. In this case the extensions may extend out of opposite sides of the strip. In an alternative form the electrodes themselves may be used as electrical connectors; and
(h) sealing the structure into a package. Since the structure will likely have a generally flat, rectangular shape it may conveniently be packaged into an envelope of plastics material. The envelope may be left open along one side during the impregnating step (f) and subsequently sealed.
A charge storage device having capacitance, time constant, power or energy density selected from a wide range, can be manufactured in this fashion by cutting the layered strip to different predetermined lengths. Additional flexibility is also readily available by changing the proportions of activated and conductive carbon, or the thickness of the coating layer that is applied onto the foil.